1. Field of the Invention
The present invention relates to a zoom optical system and an image taking apparatus using that optical system.
2. Description of the Related Art
In recent years, in a camera such as a digital camera including an image taking optical system having an electronic image sensor, miniaturization and improvement of definition have proceeded. There is an increasing demand for a cellular phone on which such camera is mounted. Accordingly, there is also a demand for miniaturization and high function of the image taking optical system for use in such camera. In such image taking optical system, a combination of optical elements is devised in order to reduce the whole length of the optical system and an outer diameter of a lens.
As zoom lenses for achieving the miniaturization, there are: a zoom lens (e.g., Japanese Patent Application Laid-Open No. 4-242709) of a two-unit type including a first lens unit having a negative refractive power and a second lens unit having a positive refractive power; a zoom lens (e.g., Japanese Patent Application Laid-Open No. 9-21950) of a three-unit type including a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power; a zoom lens (e.g., Japanese Patent Application Laid-Open Nos. 5-93866 and 2004-294910) of a three-unit type including a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a negative refractive power; a zoom lens (e.g., Japanese Patent Application Laid-Open Nos. 9-179026, 11-109230, 2004-131130, and 2004-240464) of a four-unit type constituted by disposing a field lens unit having a positive refractive power in a basic three-unit type including a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a negative refractive power and the like.
In the zoom lens described in Japanese Patent Application Laid-Open No. 4-242709, the two-unit type is adopted which includes the first lens unit having the negative refractive power, and the second lens unit having the positive refractive power. The zoom lens has a zooming ratio of about three, the whole length of the optical system is 10.3 times a maximum image height, and the system is thus comparatively small-sized.
In the zoom lens described in Japanese Patent Application Laid-Open No. 9-21950, the three-unit type is adopted which includes the first lens unit having the negative refractive power, the second lens unit having the positive refractive power, and the third lens unit having the positive refractive power.
In the zoom lens described in Japanese Patent Application Laid-Open No. 5-93866 and in a dual focal length lens described in Japanese Patent Application Laid-Open No. 2004-294910, the three-unit type is adopted which includes the first lens unit having the negative refractive power, the second lens unit having the positive refractive power, and the third lens unit having the negative refractive power. Since this type can impart a comparatively intense zooming function to the third lens unit as compared with the zoom lens described in Japanese Patent Application Laid-Open No. 9-21950, the type is very effective for reduction of the whole length of the optical system and reduction of the lens outer diameter.
In the zooming optical system described in Japanese Patent Application Laid-Open No. 9-179026, the image taking optical system for video cameras described in Japanese Patent Application Laid-Open No. 11-109230, the zoom lens described in Japanese Patent Application Laid-Open No. 2004-131130, and the optical system described in Japanese Patent Application Laid-Open No. 2004-240464, the four-unit type is adopted which includes a field lens unit having a positive refractive power in addition to the basic three-unit type that includes the first lens unit having the negative refractive power, the second lens unit having the positive refractive power, and the third lens unit having the negative refractive power.